Technical Field
The present disclosure generally relates to electronic circuits, and more particularly to at an active circuit capable of implementing a diode function.
Discussion of the Related Art
FIG. 1 shows an electronic diagram of a circuit 1 capable of implementing a diode function, that is, capable of conducting a current between a first terminal A of the circuit and a second terminal K of the circuit when the voltage between terminals A and K is positive, and of blocking the current flow between terminals A and K when the voltage between terminals A and K is negative. Such a circuit may for example be used in a system where a secondary battery is recharged from a primary battery to avoid, at the end of charge, for the secondary battery to discharge into the primary battery.
Circuit 1 of FIG. 1 comprises, connected between terminals A and K, a switch 3 having an internal resistance ron in the on state. Circuit 1 further comprises an operational amplifier 5 assembled as a voltage comparator, having a positive input connected to terminal A, a negative input connected to terminal K, and an output connected to a control node of switch 3.
Circuit 1 operates as follows. When the voltage between terminals A and K is greater than 0 V, the output of comparator 5 is at a level causing the turning on of switch 3 and, when the voltage between terminals A and K is smaller than 0 V, the output of comparator 5 is at a level causing the turning off of switch 3. Thus, when the voltage between terminals A and K is positive, circuit 1 enables a current to flow between terminals A and K, and when the voltage between terminals A and K is negative, circuit 1 blocks the current flow between terminals A and K.
FIG. 2 is a diagram showing the ideal targeted current-to-voltage characteristic of circuit 1 of FIG. 1. The axis of abscissas shows voltage V between terminals A and K and the axis of ordinates shows current I between terminals A and K. In this example, the operational amplifier is considered to be ideal, that is, it enables to control the turning on of switch 3 as soon as voltage V becomes greater than 0 V, and the turning off of switch 3 as soon as voltage V becomes smaller than 0 V. When voltage V is negative, switch 3 is off, and current I is zero. When voltage V is positive, switch 3 is turned on, and current I is determined by proportionality relation I=V/ron.
However, in practice, a comparator is never ideal, and inevitably has an offset voltage Vos between its positive input and its negative input. As a result, voltage V between terminals A and K, instead of being compared to zero, is actually compared to the value of offset voltage Vos, which causes an unwanted offset of the switching threshold of circuit 1. It should be noted that offset voltage Vos is a characteristic which, for a given comparator type, may vary according to manufacturing dispersions.
FIG. 3 is a diagram showing the real current-to-voltage characteristic of circuit 1 of FIG. 1 in two unfavorable cases. More particularly, FIG. 3 comprises a curve C1, in dotted lines, showing the current-to-voltage characteristic of circuit 1 in the case where operational amplifier 5 has a negative offset voltage Vos=Vos(min), for example, equal to −5 mV, and a curve C2, in full line, showing the current-to-voltage characteristic of circuit 1 in the case where operational amplifier 5 has a positive offset voltage Vos=Vos(max), for example, equal to 5 mV. In the first case (curve C1), switch 3 switches when voltage V reaches threshold Vos(min), and an unwanted negative current Ios(min)=Vos(min)/ron may then flow between terminals A and K. In the second case (curve C2), switch 3 switches when voltage V reaches threshold Vos(min). At the turning-off of the device, the conduction is then interrupted while a positive current Ios(max)=Vos(max)/ron still flows between terminals A and K. This may in particular cause an unwanted oscillation of the switch.
Such a shifting of the switching threshold with respect to the targeted 0-V threshold may pose accuracy problems in certain applications. Essentially, in the case of a negative offset voltage Vos, the circuit may conduct a current in the wrong direction when the voltage between terminals A and K is negative, and in the case of a positive offset voltage Vos, the circuit may prevent current from flowing between terminals A and K when the voltage between terminals A and K is positive.
As an illustration, for a resistance ron of 50 mΩ and for an offset voltage of ±5 mV, the current inaccuracy of the circuit is ±100 mA, which is far from negligible.
Further, the abrupt turning-on of switch 3 when voltage V is not strictly zero may cause current peaks. In the case where switch 3 is a MOS transistor, charge injection issues may add to the current peaks. This may pose problems of electromagnetic compatibility with neighboring systems. Further, the flowing of a non-negligible current I between terminals A and K when voltage V is negative (curve C1) may cause malfunctions in certain applications.
To overcome such disadvantages, a solution comprises attempting to decrease the offset voltage of comparator 5. Known solutions to decrease the offset voltage of a comparator may however raise other issues. Further, the provided improvement remains insufficient for certain applications.